Milky Way brims with planets

Carl Sagan would have loved it: not only are there billions and billions of stars in our galaxy, but every star may also harbour a planet. Millions of these could be like the fictional planet Tatooine in Star Wars, which orbits two stars.

About 700 extrasolar planets have been found in the Milky Way, a small number compared with the number of stars present. To find out whether such planets are truly rare or just hard to find, Arnaud Cassan of the Pierre and Marie Curie University in Paris, France, and colleagues turned to gravitational microlensing, in which one star focuses the light from a more distant star.

While other techniques are best at finding planets around nearby sun-like stars, gravitational microlensing can study any star up to 20,000 light years away.

The researchers studied six years of microlensing data from the two projects and estimated that extrasolar planets are the rule rather than the exception, with each star in the galaxy hosting an average of 1.6 planets. More specifically, 17 per cent of the stars host a Jupiter-like planet, 52 per cent have a Neptune-like planet, and 62 per cent harbour a super-Earth – a rocky planet up to 10 times as massive as Earth.

Super-Earths

The seeming abundance of rocky super-Earths lends support to the core accretion model of planet formation, in which small rocky bodies collide and clump together to grow into these objects.

"Our results suggest that Earths should be even more common than super-Earths, if the mechanism to build an Earth is similar to that of building a super-Earth," says Cassan.

Meanwhile, William Welsh of San Diego State University in California and colleagues studied 750 stars observed by NASA's Kepler satellite. Based on their findings, they reckon several million planets in our galaxy orbit two stars, like the Star Wars planet Tatooine.

"Nature seems to like forming planets. The more carefully we look, the more of them we find," says Welsh.

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